Optical transport has become an important data channel medium. From the advent of fiber-optic long distance telecommunications in the 1980's to the extensive optical fiber information distribution infrastructure investments currently being made, there has been an insatiable demand for the great bandwidth promised by optical transport.
Some of the workhorses of optical communications technology are the Wavelength Division Multiplexing (WDM) multiplexers and demultiplexers. WDM multiplexing has been used to provide multiple communication channels of transmission (MUX) or reception (DEMUX) within a single optical fiber carrying a broad wavelength signal. Multiple channels of transmission or reception are accomplished by isolation of narrow wavelength regimes within the broad, transmitted passband. If, in addition, the device used to select a narrow wavelength region, or channel, can be selectively tuned to any narrow region within the passband, then two such devices in series establish a complete optical cross-connection (or cross-switch).
Optical WDM according to the prior art is obtained using miniature diffraction gratings coupled to fibers or bundles of optical fibers. The maximum number of channels currently available in a WDM system is about 24. Using recently developed echelle grating technology, there exists the potential to increase that to 40-80 channels. Although these are impressive improvements, it would certainly be useful to produce a WDM system with over one hundred channels. Unfortunately that is not available in the prior art.
Basic liquid crystal Fabry-perot (LCFP) etalon technology has been known for some time. And it has been known to make an LCFP etalon tunable. However, prior art optical WDM systems have not usefully exploited LCFP etalon technology.
As a preliminary matter, the basic aspects of LCFP etalon technology is reviewed as follows. Referring to FIG. 1, a cross section view of a liquid crystal-filled Fabry-Perot etalon according to the prior art is illustrated. A first etalon substrate 102 and a second etalon substrate 104 are spaced apart from one another. The etalon substrates 102, 104 are typically formed of fused silica. Precision-dimensioned spacer beads 122, 124 define the spacing between the etalon substrates 102, 104. Dielectric reflector layers 106, 108 are coated onto each of respective opposed faces of the etalon substrates 102, 104.
Transparent conductor layers 110, 112 are also coated onto the substrates 102, 104. The top coating layers on each of the substrates 102, 104 are polyimide alignment layers 114, 116. After coating, the polyimide is buffed to provide alignment functionality. A liquid crystal material 130 is filled in between the substrates 102, 104.
Basic LCFP arrangements were first described in the late 1970's by W. J. Gunning and P. Yeh. For specifics, refer to Gunning and Yeh “Multiple-Cavity Infrared Electro-Optic Tunable Filter”, SPIE Proc., 202, 21-25 (1979), and Yeh and Gunning SPIE Proc., 202, pp. 2-15 (1979). Other publications that show the subsequent development of this technology are Gunning et al., “A Liquid Crystal Tunable Filter: Visible and Infrared Operation”, SPIE Proc., 268 (1981), and Maeda et al., “Electronically Tunable Liquid-Crystal-Etalon Filter for High Density WDM Systems”, IEEE Photonics Technology Lett., 2 No. 11, (1990).
By applying current to the conducting layers 110 and 112, the liquid crystal (aligned by the polyimide layer) changes its orientation relative to the axis of light passed through the system, so that the index of refraction of the material within the etalon gap is electronically tunable. Because the wavelength of light being passed by a Fabry-Perot is a function of the refractive index in the etalon gap, the device may be scanned through wavelength or positioned to a calibrated wavelength by simple voltage tuning. This simple LCFP tunable filter is here improved upon and adapted for simultaneous passage of multiple wavelengths within narrow bands in a WDM or cross-switch configuration.
It has been proposed to use a tunable liquid crystal Fabry-Perot etalon as a light modulator. For details, refer to U.S. Pat. No. 4,779,959 to Saunders. A twisted-nematic LCFP device has been described that is tunable and polarization insensitive. For details refer to U.S. Pat. No. 5,068,749 to Patel. It has also been proposed to use a nematic LCFP as a tunable filter or as a light modulator. For details refer to U.S. Pat. No. 5,111,321 to Patel and U.S. Pat. No. 5,150,236 to Patel.
Tunable liquid crystal etalons, as described in the prior art, are not useful for WDM or cross-switching. There are salient limitations of the prior art that evidence this. First, the prior art LCFP etalons have limited spectral resolution, which limits the number of possible WDM channels. Second, in response to the limited spectral resolution, the Patel '236 patent enhances the reflectivity of the dielectric coatings to greater than 95%. Although this modification of the earlier, simple designs (specifically, those of Yeh and Gunning (1979), Gunning and Yeh (1979), and Gunning et al (1981)) does enhance spectral resolution, it drastically sacrifices transmission performance. The enhanced reflectivity produces a longer photon path-length within the Fabry-Perot resonant cavity, intrinsic scattering and absorption losses are enhanced, and thus, overall LCFP etalon transmission is reduced. Furthermore, the scanning range of the LCFP etalon is not enhanced by the modifications taught by the Patel '236 Patent, and the ultimate spectral resolution remains limited by Fabry-Perot etalon substrate parallelism and surface defects. Finally, the prior art does not explicitly detail the design of a multiple channel WDM device or a cross-switch.
The spectral resolution of a Fabry-Perot filter is determined by the thickness of the resonant gap between the etalon reflecting layers. The attainable resolution is given byΔλ=(λ2\2μt)\F  (1)where λ is the sampled wavelength, μ is the refractive index of the material in the etalon gap, and t is the gap thickness. F is a quantity commonly called the finesse. Finesse depends upon the reflectivity of the dielectric coatings, the parallelism of the reflecting surfaces, and upon optical defects in the etalon glass substrates or in the medium between the plates. F is typically a value between 8-50 for plane parallel Fabry-Perot etalons.
A Fabry-Perot etalon using liquid crystal in the gap (according to the prior art) is limited to a maximum etalon gap (t) of only 30-100 microns, depending on the LC used. Larger gap spacings are not possible for the prior art LC etalon because the liquid crystal fractures if the gap exceeds 30-100 microns. In prior art, the etalon gap spacing is limited by the effective limit of the LC layer thickness. Thus, a liquid crystal etalon designed to permit larger gap spacing and hence improved spectral resolution is needed to allow the largest possible number of WDM channels.
It has been proposed to enhance the gap of an LCFP etalon for use in WDM. However, this proposal suffers from the disadvantages that it does not address cross-switching capability by parsing out the real estate of the transparent conducting layer, and that the design of the gap enhancement is mechanically unstable. For additional details, refer to U.S. Pat. No. 5,321,539 to Hirabayashi et al.
The International Telecommunications Union (ITU) has specified the wavelength bands beginning at 1528 nm (C-band) and 1884 (L-band) as telecommunications network standards (ITU standard G.692). The C-band specification is 1528 nm-1560 nm. In addition, the ITU has set 100 channels within each band as the WDM goal standard, although that number may soon be changed to 200 channels.
Thus, what is needed is a device that increases the number of WDM channels that can be isolated by existing devices, simultaneously establishes the cross-connection, and does so with a mechanically robust, solid-state device.